What Gyoung said is very true! You've probably become to used to animals and still life drawings that the human body feels completely alien to draw. You get discouraged by that because you're not used to it working out as well as animals do. But don't give up!
You say you struggle a lot with proportions. I strongly recommend taking a look at www.posemaniacs.com and the site's very own gesture drawing tool. Instead of refining the shapes and details of these drawings, try to break the images down into sections: a circle for a head, a box for the torso etc. Only draw the basics and compare them with your references.
Study people in real life too. Notice where the symmetry is. A belly button usually lines up with the elbow when someone is standing in a relaxed pose with their arms down the sides of their body. The fingertips are halfway down the upper thigh. The crotch is usually around the wrist area.
These key elements can be a very useful guide to help you get used to the human proportions. Practice is the most important element though, but I'm sure you're going to improve rapidly with that! :)